Saving drafts locally

Is it possible to force Mail to save a local copy of draft emails? It always seems to save them to the server and then when I go back to them it has to retrieve them (even if it's an email I wrote on the iPhone itself). This is a very minor thing but it causes me some annoyance at times because I have a very variable signal at work and it seems illogical for the email not to exist on the iPhone I've typed it on 10 seconds previously.